118 



ZOOLOGY. 



Fig. 80, — Schematic figures of a Sta-ui-chin. A, from 

 the oral end ; B, from one side. Ambulacra indicated 

 by rows of dots. ?■, ambulacra!; ir, interambulacral 

 areas; o, mouth; o, vent. — After Gegeubaur. 



In order to examine the external anatomy, the shell 

 should be deprived of its spines in part, meanwhile observ- 

 ing the mode of attachment of the spines, of which micro- 

 scopic sections 

 should be made. 

 The solid mouth- 

 parts, the oral 

 membrane sur- 

 rounding the five 

 sharp conical teeth 

 or "pyramids," 

 and their mode of 

 attachment to the 

 " auricles " in the 

 shell, should be thoroughly investigated, as well as their re- 

 lations to the mouth-opening and the digestive canal. The 

 shell consists of five double rows of ambulacral plates, 

 perforated for the exit of the 

 feet, and a series of five dou- 

 ble rows of interambulacral 

 plates to which- the spines 

 are attached, and of such 

 form and arrangement as to 

 give the greatest possible 

 strength and lightness to the 

 shell (Pigs. 80, 81, 82). The 

 outlet of the alimentary canal 

 is situated on the aboral 

 (abactinal) or upper end of 

 the shell, while the madre- 

 poric plate is situated upon 

 the top or end of the shell 

 (as the animal moves mouth 

 downward), being a modifica- 

 tion of one of the genital 

 plates (Fig. 81, m). There are 

 five large plates, one at each end of the interambulacral 

 zones meeting on the aboral end of the body ; in them are 

 tlie ovarian openings through which the eggs escape ; these 



Pig. 81.— Aboral end of the ehell of an 

 Echinus, with the upper end of the rows of 

 plates, a, ambulaci'al area; i. interambu- 

 lacral area; ff, i ' 

 tal plates; wa, one 



lacral area; g, genital plates; ig, intergeni- 

 of the genital plates 



formin^amadreporic plate; a;, anal opening 



spmes 



are attached are only drawn on one ambula- 



m the aboral area soiTounded by 

 plates. The tubercles to which 



the geuiti 

 the spmes 



oral and one interambulacral area ; on the 

 former are also drawn the poree through 

 which the suckers protrude.— After Gegeu- 

 baur. 



